This invention relates generally to an air cleaning device and, more particularly, to a portable air cleaning device that utilizes a replaceable filter cartridge for removing undesirable particulates from a moving stream of air.
Many air cleaning devices employ replaceable filters formed of an absorbent material that is arranged to establish a circuitous path for air flow produced by an electrically powered blower. During normal operation of such air cleaning devices, the filters progressively accumulate particles that reduce the filters permeability. Eventually, the air flow through the cleaning device becomes diminished by filter clogging to significantly reduce air cleaning efficiency. At that time, a filter must be replaced to restore efficient cleaning operation.
To assist a user in maintaining efficient cleaning operation, some air cleaning devices are equipped with indicators for monitoring the condition of a filter and indicating when it should be replaced. One such indicator consists of a transparent, vertically disposed tube having a lower end at atmospheric pressure and an upper end communicating with an internal portion of the cleaner downstream from the filter. As the filter becomes clogged with particulate matter to reduce air flow, an increased pressure drop occurs across the filter and a negative pressure is produced downstream thereof. The resultant differential pressure across the indicator tube creates an increased air flow that lifts a float member to provide an indication of a dysfunctional filter. Air cleaning devices employing filter replacement float indicators are disclosed, for example, in U.S. Pat. Nos. 3,071,914 and 5,131,932. A problem associated with float type indicators is that air flow through the indicator tube is dependent upon the magnitude of negative pressure created by the electrically operated blower. Thus, in air cleaning devices employing blowers with multiple speed motors, some form of indicator calibration is required for different blower operating speeds.
The object of this invention, therefore, is to provide an air cleaning device with an improved filter replacement indicator.